Field of the Invention
The invention relates to an automatic-machine-bondable ceramic circuit carrier, and to a process of producing such a ceramic circuit carrier.
Conventional ceramic circuit carriers, which are used as thick-film modules, have a ceramic substrate with a ceramic content of 96% by weight and a glass content of 4% by weight. To produce contact pads on the surface of the ceramic substrate, a conductive paste or gold paste with a glass content of between 1% and 10% is applied. This glass content is necessary for achieving an adhesion between the ceramic substrate and the gold paste and for forming a sintered structure of the gold paste. When the ceramic circuit carrier is fired, the glass contained in the gold paste flows out or drains off to a sufficient extent, so that the surface of the gold contact pad is sufficiently free of glass constituents. On such a surface, a gold wire can be used for bonding without problems.
Because of a wide range of advantageous properties, circuit carriers which are based on LTCC (Low Temperature Cofired Ceramics) are being increasingly used. Such a ceramic circuit carrier is known, for example, from the article xe2x80x9cKeramik-Multilayer: Technologie mit Recyclingvorsprungxe2x80x9d (Ceramic Multilayer: Technology having a Lead in Recycling) in the journal Elektronik, 3/1994, pages 30 to 33. The LTCC substrate is generally formed of from 40% to 70% ceramic fraction (Al2O3) and 60% to 30% glass fraction (SiO2) . During the firing process, the high glass content serves as a sintering catalyst and allows using a relatively low firing temperature.
The high glass content in the LTCC ceramic substrate, hinders the glass from flowing or draining off from the surface of the gold paste during firing. This results in a segregation or precipitation of glass residues on the surface of the gold paste and agglomerates inside the gold paste. Both effects prevent the uniform formation of a so-called wedge, i.e. a homogenous contact surface between the gold wire and the gold paste (contact pad), on the gold thick-film surface. This causes considerable fluctuations in the adhesive strength of the bond connection on the gold thick-film surface. In addition, the glass constituents at the gold surface result in an increase in the contamination of the capillary, i.e. the gripper tool of an automatic bonding machine. As a result, the service life of an automatic bonding machine is reduced considerably.
To ensure a sufficient process reliability during the bonding of LTCC circuit carriers, a second bond is routinely formed at the location of the wedge, in order to stabilize the first bond. As a result, the costs and the time required for producing each circuit carrier rise considerably.
Furthermore, the capillary has to apply comparatively large forces during production of the bond in order to ensure that the bond connection is secure. This presents an obstacle to further increasing the integration level on a circuit carrier with a high glass content, since it is not possible to use finer capillaries with a smaller diameter. In addition, wear is promoted.
Conductive, glass-free molybdenum structures on a ceramic substrate are described in the IBM Technical Disclosure Bulletin, 1982, Vol. 25, No. 5, page 2275. The ceramic substrate is formed of aluminum containing about 10-20 percent by volume of glass powder. To prevent the glass from penetrating into the molybdenum, palladium is admixed with the molybdenum paste. The molybdenum structures of this HTCC (High Temperature Cofired Ceramic) substrate cannot be bonded. Publication No. JP 07157812 A in Patent Abstracts of Japan relates to a metallized composition which has an improved adhesion with respect to a ceramic substrate (alumina substrate). To this end, pulverized glass and copper oxide are added to a metal paste which is substantially composed of copper powder. An alumina substrate for which the metallized composition is to be used generally has a ceramic content of approximately 96% and a glass content of only 4%. A bonding with gold wires on the conductive structures, which are formed by the metallized composition, is not possible.
It is accordingly an object of the invention to provide a process for producing an automatic-machine-bondable ceramic circuit carrier which overcomes the above-mentioned disadvantages of the heretofore-known processes of this general type and which uses a ceramic substrate with a high glass content. It is also an object of the invention to provide a corresponding ceramic circuit carrier, which has a reduced glass concentration on the surface of contact pads, which are thus well suited for a fully automated bonding.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for producing an automatic-machine-bondable LTCC circuit carrier, which includes the steps of:
providing a ceramic substrate having a glass content of at least 30 percent by weight;
applying a conductive paste having a glass content of less than 1.0 percent by weight on the ceramic substrate for forming contact pads; and
melting glass out of the ceramic substrate and causing an adhesion between the conductive paste and the ceramic substrate by firing the ceramic substrate and the conductive paste for forming an automatic-machine-bondable LTCC circuit carrier.
According to the invention, the glass content in the conductive paste is reduced considerably. The glass content in the ceramic substrate plays the decisive role in producing the adhesion between the conductive paste and the ceramic substrate. A homogeneous distribution of glass agglomerates in the conductive paste is avoided.
The glass content in the ceramic substrate is at least 30 percent by weight. Preferably, the glass content in the ceramic substrate is at most 60 percent by weight.
A suitable conductive paste is in particular a paste made from a precious metal, such as gold, silver, palladium, platinum and their alloys. The precious metal content should be at least 98.9 percent by weight, and preferably at least 99.0 percent by weight. The glass content should be less than one percent by weight, preferably less than 0.3 percent by weight, and particularly preferably at most 0.1 percent by weight.
According to a preferred feature of the invention, the conductive paste is entirely free of crystalline glass. Any glass fractions contained in the conductive paste are then amorphous glass.
The sintering behavior of the conductive paste can be adapted to the flow properties of the glass in the ceramic substrate by the addition of sinter-active additives, so that despite the reduction in the glass content in the conductive paste a sufficient number of adhesion centers are formed between the ceramic substrate and the conductive paste.
The optional addition of sinter-active additives to the conductive paste ensures a particularly homogeneous sintering of the conductive paste. Suitable additives are metals or metal oxides, such as for example rhodium (Rh), cadmium (Cd), antimony (Sb), iron (Fe) and bismuth oxide (Bi2O3). The content of one or more sinter-active additives may amount to approximately 0.2 to 2.5 percent by weight, and is preferably between 0.3 and 1.1 percent by weight.
With the objects of the invention in view there is also provided, an automatic-machine-bondable ceramic circuit carrier, including:
a ceramic substrate having a glass content of at least 30 percent by weight;
a contact pad disposed on the ceramic substrate, the contact pad being formed of a conductive paste having a glass content of less than 1.0 percent by weight prior to the ceramic substrate and the conductive paste being fired; and
an adhesion-promoting glass disposed between the ceramic substrate and the conductive paste.
Due to the pure surface of the conductive paste, which is not disrupted by inhomogeneous inclusions of glass and oxide agglomerates, the ceramic circuit carrier according to the invention allows a uniform formation of wedges. The bonding operation on such a circuit carrier has a high level of process reliability, so that fully automatic bonding machines can be used without problems. There is no need for the contact pads which are to be bonded to be visually checked for glass agglomerates by an operator. Nor is there any need for a second bond (security bond) for the production of an electrical connection. In a preferred embodiment of the invention there is accordingly only one bond connection per contact pad.
Due to the lack of glass agglomerates on that surface of the contact pads which is remote from the ceramic substrate, the contamination to the capillary of an automatic bonding machine is reduced considerably, and its service life is increased substantially.
Due to the homogeneous surface of a contact pad, the bonding forces which have to be applied by the capillary of an automatic bonding machine are low, and consequently the diameter of a capillary can be reduced. Due to the finer structure, the integration level on the ceramic circuit carrier can be increased. The diameter of a bonding wire can be reduced. Instead of a range from 30 xcexcm to 60 xcexcm, a range from 15 xcexcm to 60 xcexcm is now suitable for the diameter of the bonding wire.
Moreover, the low level of contamination of the capillary and the relatively low bonding forces which have to be applied increase the service life of the capillaries such that the service life is multiplied. It is therefore possible to perform a virtually uninterrupted bonding process.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for producing an automatic-machine-bondable ceramic circuit carrier, and in a corresponding automatic-machine-bondable ceramic circuit carrier, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.